You shouldn't need to be a member of the crontab group to have a personal crontab. It looks like you are still on the original Linux that came with the Plug. If so, be aware that many (all?) of the setuid/setgid programs in that load did not have their SUID/SGID bits set correctly. For instance /bin/su was not SUID root, so you could not "su" as a regular user.

I suspect the /usr/bin/crontab permissions are similarly incorrectly set. Here's what they should look like:

-rwxr-sr-x 1 root crontab 27492 2009-05-12 17:55 /usr/bin/crontab

(The size and date will differ on your load.)

If your /usr/bin/crontab does not have "crontab" as its group, and does not have the SGID bit set (the 's' in "-rwxr-sr-x") that is your problem. You can fix this by "# chgrp crontab /usr/bin/crontab; chmod g+s /usr/bin/crontab". But, I think you'll find this is the tip of the iceberg. FWIW, the alpha-6 root fs seems to be correctly configured.

I can attest to the fact that crontabs work under the alpha-6 load, as well as some other things that are broken under the load that comes with the Plug.

If you want to continue diagnosing your problem, I'd suggest looking at /var/spool/cron/crontabs/: Does this directory exist? Are the permission and owner/group correctly set? Here's what it should look like: